docs: def get_backend(self):
          """ Returns the associated cache backend. """
          try:
              cache = caches[machina_settings.ATTACHMENT_CACHE_NAME]
          except InvalidCacheBackendError:
              raise ImproperlyConfigured(
                  'The attachment cache backend ({}) is not configured'.format(
                      machina_settings.ATTACHMENT_CACHE_NAME,
                  ),
              )
          return cache